The present invention relates to a stimulable phosphor sheet employable in a radiation image recording and reproducing method utilizing stimulated emission produced by a stimulable phosphor.
As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screens, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and is practically employed. The method employs a stimulable phosphor sheet comprising a stimulable phosphor layer (i.e., radiation image storage panel) provided on a support, and the procedure of the method comprises the steps of causing the stimulable phosphor of the stimulable phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as xe2x80x9cstimulating raysxe2x80x9d) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The stimulable phosphor sheet thus processed is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly employed.
In general, a support and a protective film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The stimulable phosphor sheet containing no binder can be formed by the deposition process or firing process. Further, the stimulable phosphor sheet comprising agglomerated phosphor soaked with a polymer is also known. For the aforementioned method, any types of the stimulable phosphor sheets are employable.
The radiation image recording and reproducing method is often used in radiography for medical diagnosis. In that case, it is especially desired to obtain a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Therefore, the stimulable phosphor sheet is required to have high sensitivity and to provide a radiation image of high quality.
The sharpness of tradiation image is mainly affected by diffusion or scattering of the stimulating rays in the stimulable phosphor sheet. The procedure for reading the latent image comprises the steps of sequentially scanning a beam of the stimulating rays on the surface of the stimulable phosphor sheet to induce the stimulated emission, and successively detecting the stimulated emission. If the stimulating rays diffuse or scatter (horizontally in particular) on the plane of the phosphor sheet, it excites the phosphor not only at the target spot but also in its periphery. Consequently, the stimulated emission emitted from the target position is collected together with that from the periphery. Such contamination of the emissions impairs the sharpness of the resulting radiation image.
For avoiding the diffusion or scattering of the stimulating rays, it has been proposed to divide the plane of the stimulable phosphor sheet into small sections (cells) with a partition capable of reflecting the stimulating rays.
Japanese Patent Provisional Publication No. 59-202100 discloses a stimulable phosphor sheet having a honey-comb structure consisting of many small cells filled with a stimulable phosphor. The phosphor sheet comprises a substrate and a stimulable phosphor layer provided thereon, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 62-36599 discloses a stimulable phosphor sheet employing a support provided with many hollows regularly arranged on one surface. The hollows are filled with a stimulable phosphor, and the ratio of depth to diameter of each hollow is 3.5 or more.
Japanese Patent Provisional Publication No. 2-129600 discloses a stimulable phosphor sheet (i.e., radiation image storage panel) employing a support plate having many holes vertically bored and filled with a stimulable phosphor.
In these known stimulable phosphor sheets employing a support or substrate provided with many cells (i.e., holes or hollows) incorporated with a stimulable phosphor, a part of the support or substrate serves as a partition for reflecting the stimulating rays to prevent diffusion of the stimulating rays.
In the stimulable phosphor sheet having the cells, the partition preferably has a small thickness so that the stimulable phosphor-incorporated area is formed as large as possible. However, it is necessary to incorporate a light-absorbing material into a partition if the partition is made to have a small thickness, maintaining the light-intercepting property. The incorporation of the light-absorbing material into the partition decreases efficiency of recovery of the stimulated emission. If the thickness of the partition is increased, the amount of stimulable phosphor which can be incorporated into the stimulable phosphor sheet decreases and hence an enough amount of radiation energy is absorbed by the phosphor sheet. It can be considered to increase the thickness of the stimulable phosphor layer so as to incorporate an enough amount of the stimulable phosphor. However, the radiation energy hardly reaches the stimulable phosphor in the bottom portion in the phosphor layer having the increased thickness. Moreover, the stimulated emission produced in the bottom portion is not efficiently recovered from the surface of the stimulable phosphor sheet. For these reasons, there is not known a stimulable phosphor sheet that has a cell structure and gives a reproduced radiation image of satisfactorily high quality.
Accordingly, the present invention has an object to provide a stimulable phosphor sheet which gives a reproduced radiation image of high quality, particularly, in its sharpness.
The invention further has an object to provide a stimulable phosphor sheet that has high sensitivity and gives a reproduced radiation image of high quality, particularly, in its sharpness.
The present inventors have made studies on the cell structure of the stimulable phosphor sheet and discovered that the partition should have a high reflectance to the stimulating rays by shortening its light-scattering length and should have a low absorbance for the stimulated emission by prolonging its light-absorption length for the stimulated emission, so that the diffusion of the stimulating rays on the plane of the phosphor sheet can be efficiently reduced to increase sharpness of the obtained radiation image and the decrease of emission efficiency can be effectively obviated by suppressing absorption of the stimulating rays as well as the stimulated emission. The inventors have further discovered that the stimulable phosphor-incorporated area should show a high transmittance for the stimulating rays by prolonging its light-scattering length and should have a low light-absorbance for the stimulated emission by prolonging the light-absorption length for the stimulated emission, so that the stimulating rays can reach the deep area of the stimulable phosphor sheet and the stimulated emission produced in the deep area can be efficiently recovered. Thus designed stimulable phosphor sheet gives a reproduced radiation image of high quality. Moreover, the stimulable phosphor sheet which has such characteristics and a large thickness gives a reproduced radiation image of increased high quality, increasing its sensitivity, namely, a ratio of absorption of radiation energy.
The invention resides in a stimulable phosphor sheet for a radiation image recording and reproducing method comprising the steps of recording a radiation image as a latent image, irradiating the latent image with stimulating rays to release stimulated emission, and electrically processing the emission to reproduce the radiation image, comprising partitions that divide the phosphor sheet on its plane to give plural sections, and stimulable phosphor-incorporated areas in the sections wherein the stimulable phosphor-incorporated areas have a light-scattering length of 20 to 100 xcexcm and a light-absorption length of 1,000 xcexcm or longer, while the partitions have a light-scattering length of 0.05 to 20 xcexcm and a light-absorption length of 1,000 xcexcm or longer, under the condition that a ratio of the light-scattering length of the stimulable phosphor-incorporated areas to that of the partitions is 3.0 or more.
The invention further resides in a radiation image recording and reproducing method comprising the steps of recording on a stimulable phosphor sheet of the invention a radiation image as a latent image, irradiating the latent image with stimulating rays to release stimulated emission, and electrically processing the emission to reproduce the radiation image.
In the present invention, the term of light-scattering length indicates a mean distance in which a light travels straight until it is scattered, and therefore a shorter light-scattering length means that the phosphor layer or partition highly scatters a light. The term of light-absorption length indicates a mean free distance in which the stimulated emission is absorbed, and therefore a longer light-absorption length means that the phosphor layer or partition shows a lower light absorbance.
The light-scattering length and light-absorption length can be determined by calculation according to Kubeluka-Munk theory. The details are given below.
First, three or more film samples are prepared. Each film sample has a different thickness, but made of the same material of the partition or the stimulable phosphor-incorporated area. The thickness (xcexcm) and the diffuse transmittance (%) of each film sample are then measured. The diffuse transmittance (%) can be measured by means of a spectrophotometer equipped with an integrating sphere. In the below-described examples of the present specification, an automatic recording spectrophotometer (U-3210, manufactured by HITACHI, Ltd.) equipped with an integrating sphere of 150 xcfx86 (150-0910) is used. The diffuse transmittance should be measured at a wavelength corresponding to the maximum peak (for instance, 600 nm) of the stimulation spectrum of the stimulable phosphor in the stimulable phosphor-incorporated area, or at a wavelength corresponding to the maximum peak (for instance, 400 nm) of the stimulated emission spectrum of the stimulable phosphor.
From the thickness (xcexcm) and the diffuse transmittance (%) obtained in the above-mentioned measurements, the light-scattering length and light-absorption length are calculated by the below-mentioned formulas which are derived from Kubeluka-Munk theory. For instance, the formulas can be easily derived, under the boundary condition of the diffuse transmittance (%), from the-formulas 5.1.12 to 5.1.15 described in xe2x80x9cKeikotai Handbook [in Japanese, Handbook of phosphor]xe2x80x9d, published by Ohm-sha, 1987, pp.403.
In the beginning, light strength distribution I(Z) is calculated using a thickness (d xcexcm) of a phosphor sheet, a reflectance (d0) of the reflection sheet, a light-scattering length (1/xcex1) of the sample film, and a light-absorption length (1/xcex2) of the sample film. I(Z) is then divided into i(Z) for a component advancing from the upper surface of the phosphor sheet and j(Z) for a component advancing from the lower surface to the upper surface of the phosphor sheet. Therefore, the relationship is written as follows:
I (Z)=i(Z)+j(Z)
Increase or decrease of strength by scattering/ absorption at an optional depth in a film having a small thickness dz can be obtained by solving the following simultaneous differential equations according to Kubeluka-Munk theory:
di/dz=xe2x88x92(xcex2+xcex1)i+xcex1jxe2x80x83xe2x80x83(1)
di/dz=(xcex2+xcex1)jxe2x88x92xcex1ixe2x80x83xe2x80x83(2)
Assuming the conditions of xcex32=xcex2(xcex2+2xcex1), "xgr"=(xcex1+xcex2xe2x88x92xcex3)/xcex1, xcex7=(xcex1+xcex2+xcex3)/xcex1, and K and L for integration constants, the general solutions for i and j of the simultaneous equations are described below:
xe2x80x83i(z)=Kexe2x88x92xcex3zz+Lexcex3z
j(z)=K"xgr"exe2x88x92xcex3z+Lxcex7excex3z
The light transmittance T of a phosphor sheet having a thickness d is described as follows:
T=i(d)/i(0)
Assuming that there is no returning light (namely, j(d)=0), a light transmittance T of the phosphor sheet alone is calculated in terms of a function of the thickness d, as follows:
T(d)=(xcex7xe2x88x92"xgr")/(xcex7excex3zxe2x88x92"xgr"exe2x88x92xcex3z)xe2x80x83xe2x80x83(3)
The light transmittance data measured by means of a spectrophotometer and data of the phosphor sheet are processed according to the equation (3) and fitted by a method of least squares, to give an optimum 1/xcex1 and 1/xcex2. Thus, the light-scattering length as well as the light-absorption length are determined.
The preferred stimulable phosphor sheets of the invention are given below.
(1) The stimulable phosphor-incorporated areas comprise stimulable phosphor particles and a binder.
(2) The stimulable phosphor particles in the stimulable phosphor-incorporated areas have a volume in the range of 40% to 90% of the phosphor-incorporated area, and the phosphor-incorporated areas have a void volume of 0 to 20%.
(3) The partitions of the stimulable phosphor sheet comprise low light-absorbing fine particles and a binder.
(4) The low light-absorbing fine particles in the partitions have a volume in the range of 30% to 90% of the partition.
(5) The low light-absorbing fine particles have a particle size of 0.01 to 5.0 xcexcm.
(6) The low light-absorbing fine particles are fine particles of alumina.
(7) The partitions of the stimulable phosphor sheet further comprise voids.
(8) The partitions of the stimulable phosphor sheet further comprise voids in a volume of 10% to 70%.
(9) A ratio of refractive index of the low light-absorbing fine particles to that of the voids in the partitions is in the range of 1.1 to 3.0.
(10) The partitions further comprise stimulable phosphor particles.
(11) The partitions are dyed with a coloring material which absorbs the stimulating rays, while absorbs no stimulated emission.
(12) The stimulable phosphor sheet has a thickness in the range of 50 to 1,500 xcexcm.